The invention relates to electronic communications, and more particularly to CDMA-based coding, transmission, and decoding/synthesis methods and circuitry.
Code division multiple access (CDMA) coding has been extensively used in such applications as cellular and satellite communications. CDMA signals increase the spectrum required for the transmission at a particular data rate by modulating each data symbol with a spreading code having a rate larger than the data rate. The same spreading code is used for each data symbol. Typically, the spreading code comprises of a few tens or a few hundreds of elements, called chips. To decrease the correlations among spreading codes assigned to different users, and thereby reduce the interference among different users, the data stream after spreading is typically scrambled with a pseudo-noise (PN) code that is generated serially and cyclically and has a larger period than the spreading code. Examples of such CDMA signal spreading are the schemes used by the IS-95/CDMA2000 and 3GPP systems.
The 3GPP system employs a 3.84 MHz bandwidth (a chip duration of 260 ns), and the spreading code length applied to each data symbol may vary from 4 chips to 256 chips. The number of chips per symbol is the spreading factor (SF), and SF stays constant during a 10 ms duration frame. However, the spreading factor may change from frame to frame. Each frame is partitioned into 15 time slots (0.667 ms duration) with each time slot consisting of 2560 chips, and thus the number of data symbols per slot ranges from 10 to 640 and per frame ranges from 150 to 9600.
3GPP allows for transport channels with differing data rates and formats as specified by the Transport Format Control Indicator (TFCI) field, but 3GPP also allows for omission of the TFCI with instead only a (previously-negotiated) set of possible (candidate) transport block lengths known by the decoder. In this situation a decoder may determine the transport block length by decoding for all possible candidate block lengths and selecting one of the decodings. In particular, FIGS. 4–5 illustrate a 3GPP blind transport format detection (BTFD) method which uses the CRC bits (attached at the end of a transport block prior to convolutional encoding) as follows. First the received stream of symbols are Viterbi decoded up to the end of the shortest of the candidate block lengths. Viterbi decoding includes state metric updatings with add-compare-select (ACS) computations for each received symbol plus recording in the trace-back buffer which detected transition (0/1 input to the convolutional encoder) was used for each state metric update; this uses a buffer for current state metrics plus a buffer for the streams of inferred 0/1 inputs (the trace-back buffer). Next, the state metric for state 0 at the end of this shortest block is checked for reliability, and if reliable, then the trellis path ending at this state 0 is traced back to the beginning. The traced-back path provides a decoding for the shortest block length which can be error checked with its presumed CRC bits. Second, the Viterbi decoding is continued from the end of the shortest block to the end of the second shortest block. Again, the state metric for the state 0 at the end of the second shortest block is checked for reliability, and if reliable, then the trellis path ending at this state 0 is traced back to the beginning. This traced-back path provides a decoding for the second shortest block length which can be error checked with its presumed CRC bits. Similarly for the remainder of the block lengths; and the best (if more than one) of the error-free decodings is selected. This method uses a logarithm function in the reliability check of the state metric for the state 0.
The known method has problems including large trace-back buffers and complex logarithm computation.
Black et al, A 1-Gb/s, Four-State, Sliding Block Viterbi Decoder, 32 IEEE J. Solid State Cir. 797 (1997) describes a Viterbi decoder using a sliding decoding block of length less than the entire block to decode.